The NAIS instruments have either a 3-blower of 4-blower flow system. In the 3-blower system there one sampleflow blower for the whole instrument. In the 4 blower version, both columns have a separate sampleflow blower, so the columns work completely independently.
All the flowrates – sheathflow for both polarities and sampleflow (either single or separate) – are measured using Venturi tubes and differential pressure sensors. The sensors are calibrated at Airel.
The blowers are automatically adjusted using a software PID controller based on the flow sensors and barometric sensor so that the mobility analysis and sampling volume flowrate remain constant.
The sheath air of the mobility analyzer is taken from the exhaust air after it has been cleaned with the sheath filter. A simple electric filter is used as it is necessary to remove only the charged particles from the air that would be detected by the analyzer. Both analyzer columns have a separate sheath air filter.
In most instruments the sheath filter also contains a corona needle to charge the particles to the polarity that is not detected by the respective analyzer. In newer instruments the needle has been left off. The electrical capacitance of the filter is sufficient to clean the air. In latest instruments the internal filter design has been updated to increase the capacitance i.e. the effectiveness of the filter.
The filter voltage is typically about 7 kV. The voltage is high. When the filter is dirty and for example insects or fibers are deposited in the filter then there is a chance of spontaneous corona of either polarity. Especially if the corona discharge produces ions in the same polarity as is measured by the analyzer then the instrument will not measure correctly. See Cleaning sheath filters.
